radianA radian is a unit of angular measure that is equal to the angle subtended at the center of a circle by an arc equal in length to the radius of the circle. A radian is about 57.2957° (1 degree = 0.0174532 radians). There are 2π radians in 360°.

radicalA radical is a symbol √ that is used to indicate the square root or nth root of a number.

radiusThe radius is the distance from the center of a circle (or a sphere) to the edge.

ratioA ratio is the relationship between two numbers, denoted by division. For example, if the ratio of boys to girls at a school is 4 to 5, this can be written as 4/5 or 4:5.

rational numberA rational number is a number that can be expressed in the form a/b, where a and b are integers and b≠0. For example, 0, 1/2, and -41 are rational numbers. Each rational number corresponds to a unique point on the number line (but most points on the number line do not correspond to a rational number -- they correspond to an irrational number).

rayA ray (also called a half line) is the portion of a straight line that extends infinitely far in one direction from a point.

Roman numeralsThousands of years ago, in ancient Rome, the Romans used a system of numbers that we call Roman numerals. In this system, I=1, V=5, X=10, L=50, C=100, D=500, and M=1,000.

Roman numerals express numbers as sums and differences. For example, 6 is VI (five + one), but nine is IX (ten - one). In general, when a smaller Roman numeral follows a larger numeral, you add the numbers (for example, XII is ten + one + one = twelve). When a larger numeral follows a smaller one, you subtract the numbers (for example, IV is five - one = four, and XL is 50 - 10 = 40).

rootAn nth root of a number is a number that, when multiplied by itself n times, results in that number. For example, the number 4 is a square root of 16 because 4 x 4 equals 16. The number 2 is a cube root of 8 because 2 x 2 x 2 equals 8.

roundingWhen a number is rounded (or rounded off), it is approximated by eliminating the least significant digits. Whole numbers can be rounded to the tens place, hundreds place, thousands place, and so on. Decimals can also be rounded, estimating the number to the nearest tenth, hundredth, thousandth, and so on. Rounding is used to make a number easier to work with. For example, if you know that there are 496 students in your school, you can say that there are approximately 500 students in your school.

rulerA ruler is a long, straight object that is used for measuring distances or drawing straight lines.